1. Field of the Invention
The invention relates to a method for transmitting data in a packet-oriented communications network, in particular of a motor vehicle, in which data packets can be transmitted in the communications network without a guaranteed transmission time for the data packets (also called background data traffic) and with a guaranteed maximum transmission time for the data packets. The packet-oriented communications network may be, in particular, a network according to the Ethernet AVB standard.
2. Description of the Prior Art
The method with a guaranteed maximum transmission time for the data packets provides for a certain quality of service, in particular a maximum transmission time, to be reserved along the communication connection, that is to say the route of the data packets in the communications network, before the data transmission using a reservation request from the sender of the data to the recipient of the data. This quality of service is guaranteed only after a confirmation of the reservation request for the data stream transmitted on the communication connection between the sender and recipient in the network, with the result that the data transmission with a guaranteed transmission time for the data packets is started only after the sender has received confirmation of the reservation request. After the reservation request has been forwarded via switches provided in the communication connection between the sender and recipient, the positive confirmation of the reservation request is finally emitted by the recipient intended for the data.
As parameters of the requested quality of service, the reservation request preferably comprises the data rate of the data stream, which the sender would like to provide for the transmission of the data, and the latency, that is to say the maximum transmission time, in the form of a quality of service class which specifies, inter alia, the maximum transmission time.
The background data traffic transmitted without a guaranteed transmission time for the data packets does not require any advance reservation. Therefore, for data packets transmitted in this manner, no reservation request is emitted in advance in the packet-oriented communications network.
In a motor vehicle, in addition to a temporally guaranteed transmission time for data packets, the start-up time of the network also plays an important role within the scope of many applications. The time which is required before it is possible to transmit data via this network after the network has been started is referred to as the start-up time. In this case, the starting point is a state in which the network was previously inactive. All required configurations, synchronizations and further starting mechanisms must then be concluded. In addition to starting the respective control device, the process of starting up a network therefore also includes the process of starting the network controller responsible for the network and setting up or initializing communication with the further network subscribers in the packet-oriented communications network (also referred to as network for short). In this case, an important critical point is the temporal synchronization of the network subscribers which only enables communication of a time-controlled network such as MOST, FlexRay, and Ethernet according to the AVB standard.
Conventional motor vehicle communications networks are typically MOST and FlexRay. They enable a start-up time of approximately 100 ms. In the case of FlexRay, this means that application messages can already be sent after 100 ms. In the case of MOST, this is then typically also followed by a determination of the network configuration, with the result that the application messages can typically be sent only after 500 ms. Only then is it possible to transmit data, for example a synchronized data stream, as part of a data stream.
During conventional use of the communications network in a motor vehicle, the start-up of the network can usually already be started when the parked vehicle is unlocked using the remote control (keyless entry) since it can be assumed that the driver either moves off or otherwise accesses functions of the motor vehicle which require network communication after getting into the vehicle. Due to the time needed to get into the vehicle, an extended start-up time on account of said reservation requests in the network is not a major problem.
However, situations in which the driver of a vehicle is sitting in a switched-off vehicle in a completely inactive manner for a relatively long time, for example because he is sleeping, and then suddenly starts the vehicle and moves off problematic. Before starting the vehicle, all control devices were inactive due to the relatively long time of inactivity. After starting the car, the warning tone of the parking aid must be audible in fewer than two seconds and/or the video image from the rear view camera must be visible in fewer than two seconds according to the conventional requirements, for example.
When transmitting audio and/or video signals and control data, synchronized transmission in the data network is important, which synchronized transmission is possible, for example, with the transmission via the Ethernet according to the AVB standard, as is currently provided for transmitting audio and video data. Due the extensive standardization and flexible configuration of the Ethernet AVB standard, there is interest for use in motor vehicles in which, after standardization in 2011, it is currently used but not yet to a significant extent. An important advantage of the Ethernet AVB standard is the assurance of quality of service (QoS) for the data to be transmitted if it are intended to be transmitted as streaming data. As a result of the stream reservation protocol MSRP (Multiple Stream Reservation Protocol) of the IEEE 802.1Qat standard, resources of the communications network and, in particular, of the switches which forward the individual data packets from the sender to the recipient must therefore be reserved in advance. Such reservation can ensure that the data transmitted using the IEEE 1722 or IEEE 1733 transport protocol arrive at the recipient with a guaranteed maximum delay.
This guarantee is part of the quality of service. The above-mentioned transport protocols ensure a maximum delay of the data packets when transporting the data or streaming data via the communications network. The arrival time cannot be determined without using these protocols or equivalent protocols. This arrival time is particularly important for audio and video data, for example. This does not play a significant role in the case of background data traffic or less important, non-time-critical data or data that does not have a temporal relationship with data transmitted using other data packets, with the result that no reservation is required for this data. Accordingly, no reservation requests are transmitted for this data either.
In contrast, if data, which is intended to be transmitted in a guaranteed transmission time in order to only enable or ensure particular applications are transmitted, the process running according to the Ethernet AVB standard cannot be circumvented or significantly accelerated. If a reservation request fails, no data at all can initially be transmitted as part of the Ethernet AVB standard until the transmission with the desired quality of service has been reserved. With this quality of service guarantee of data transmission, which is comparatively complicated in comparison with other communications networks, account is taken of the fact that the Ethernet as such is a very flexible network and in use is therefore highly suitable for a multiplicity of types of communication. However, in contrast to the conventional networks in motor vehicles, for example FlexRay or MOST, the start-up of the network, that is to say the time until possible transmission of data with a guaranteed quality of service (QoS), is considerably longer. In addition, the request for resources, which is dynamically configured by the reservation request, in the communications network along a desired communication connection may also result in a refusal if the desired quality of service cannot be achieved. This is synonymous with saying that data transmission is not possible.
For this reason, the MOST and FlexRay communications networks are used in networks of a motor vehicle for data transmissions that require a certain quality, which communications networks can be implemented in this respect with considerably faster reaction times for guaranteeing a quality of service when transmitting time-critical, for example real-time-critical, data on account of their rigid network topology which is firmly predefined in the motor vehicle. The Ethernet AVB standard which is also desirable for use in the motor vehicle on account of its flexibility competes with these two communications networks usually used in motor vehicles and, if used in the motor vehicle, for example if replacing the MOST and/or FlexRay communications networks, should also support at least their quality requirements with respect to the latency times and the message jitter for the transport of data.
The static part of the data transport in a FlexRay network, for example, is fully planned before the network is started, that is to say the data transport is already defined at the time of design. As a result, the communication in the FlexRay network can be predicted at any time. This is stipulated once before the production of an automobile, for example its line or special model, and lasts for the service life of the vehicle, but at least until a software update of all network subscribers in the vehicle's communications network.